The salient advantage of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her location. For example, the user might be interested in telling a remote party where he or she is or, alternatively, the user might seek advice in navigation.
In addition, there are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location.
There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from signal measurements that are reported by the wireless terminal. The reported measurements are of signals measured by the wireless terminal that are transmitted by one or more base stations and, in some cases, by Global Positioning System (GPS) satellites. In order for these techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular estimation technique. Some of these techniques work well even in environments where the measured strengths of the different signals vary significantly, such as where signal obstructions are present, including natural obstructions such as mountains and artificial obstructions such as buildings.
There are also techniques in the prior art for estimating the elevation of a wireless terminal. Some of these techniques rely on the relationship between barometric pressure, PA, and elevation, ZA, in which PA decreases logarithmically with ZA, according to the formula:
                              Z          A                =                              -                          H              S                                ⁢                      ln            ⁡                          (                                                P                  A                                                  P                  0                                            )                                                          (                  Eq          .                                          ⁢          1                )            wherein                P0 is the reference atmospheric pressure, and        HS is the scale height of the atmosphere, which is the elevation at which the atmospheric pressure has decreased to e−1 times its value at mean sea level (e.g., approximately 8400 meters).Scale height is well known in the art and is a function of, among other factors, the temperature of the air.        
It is well known in the art how to estimate the elevation of an object—such as an airplane—using Equation 1. Aircraft altimeters have used this technique for decades, and it is well known to be highly accurate. Furthermore, it is well known in the art how to estimate the elevation of a wireless terminal using Equation 1, in which barometric pressure measurements made by the wireless terminal can be used.
Estimates of elevation based on barometric pressure, however, can be inaccurate for a variety of reasons. One reason is related to a phenomenon known as the “stack effect.” The stack effect relates to the movement of air into, within, and out of structures. This movement of air results in part from air buoyancy, which can occur from a difference in temperature between inside the structure and outside. The greater the difference in temperature, the greater the buoyancy force and the greater the stack effect, at least at certain elevations within the structure. In general, the stack effect is common in tall buildings and other structures.
The stack effect can cause the scale height inside of a structure to be different from the scale height outside. This is mainly because scale height is dependent upon air temperature, and the inside and outside air temperatures are often significantly different from each other. As can be seen in Equation 1, an incorrect value of the scale height HS can result in an inaccurate estimate of elevation.